Electron energy spectrometers, such as Auger electron spectrometers and photoelectron spectrometers, analyze energies in the manner described below. A typical structure of an Auger electron spectrometer is shown in FIG. 8. Electrons ejected from the sample 41 are analyzed by a cylindrical sector and then detected by the single detector 42. An electron energy spectrum is derived using the output from the detector. In the above analyzer with a single detector, electrons which do not agree with an analyzing condition are disregarded so that the analyzer is an efficient system.
It may be contemplated to use a multidetector arrangement in order to enhance the sensitivity. A multidetector spectrometer uses juxtaposed detectors for detecting energy-analyzed electrons. Each time a sweep of the analyzer is made, electrons of different energies are simultaneously counted by the plural detectors. The counted values are summed up for each different energy value. In this way, the amount of information gathered per unit time is increased.
This is described in further detail by referring to FIGS. 9(a), 9(b) and 9(c). A plurality of detectors 51-53 are juxtaposed so as to be regularly spaced from each other. For the sake of simplicity, only three detectors are shown to be arranged. After the first sweep, as shown in FIG. 9(a), the detector 51 detects electrons of energy E.sub.-2 and produces a total count of I.sub.-2 (1). At the same time, the detector 52 counts electrons of energy E.sub.-1 and produces a total count of I.sub.-1 (1). The detector 53 detects electrons of energy E.sub.0 and produces a total count of I.sub.0 (1). After the second sweep, as shown in FIG. 9(b), the detector 51 detects electrons of energy E.sub.-1 and produces a total count of I.sub.-1 (2). The detector 52 counts electrons of energy E.sub.0 and produces a total count of I.sub.0 (2). The detector 53 counts electrons of energy E.sub.+1 and produces a total count of I.sub.+1 (2). After the third sweep, as shown in FIG. 9(c), the detector 51 counts electrons of energy E.sub.0 and produces a total count of I.sub.0 (3). The detector 52 counts electrons of Energy E.sub.+1 and produces a total count of I.sub.+1 (3). The detector 53 counts electrons of energy E.sub.+2 and produces a total count of I.sub.+2 (3). Data about these total counts are stored in a buffer memory or the like.
We see now more specifically the function of the multichannel detection. Paying attention to the detector 53, we see that the detection energy is swept in the same way as in the prior art electron energy spectrometer equipped with only a single detector. The energy detected by the detector 53 is increased such as E.sub.0, E.sub.1, E.sub.2 . . . . Also, in the multichannel detection system, electrons which are discarded in the single channel detection system are detected simultaneously for counting by the other detectors 51 or 52. After repeating these sweeping operations, the total counts I.sub.0 (1), I.sub.0 (2), I.sub.0 (3), . . . , I.sub.0 (i) (where i detectors are provided) obtained from the detectors for the electrons of energy E.sub.0, for example, are summed up. In this way, the number of electrons counted per unit time can be increased compared with the prior art techniques. That is, the sensitivity can be improved.
However, as can be understood from the principle of the multidetector method, this method cannot be employed unless the following two requirements are met:
(1) The energy resolution .DELTA.E (the analyzer pass energy, in other words) is kept constant. PA1 (2) The energy increment, E.sub.inc, in an energy sweep must be an integral multiple of the difference of the energies detected by successive detectors.
In other words, if these two requirements are not satisfied, the values of electron energy detected as E.sub.0 by individual detectors during a sweep do not agree exactly with each other in the example shown in FIGS. 9(a), 9(b) and 9(c). Consequently, the data obtained by the summation does not correctly correspond to the energy E.sub.0. This problem is discussed further below.
Generally, in an electrostatic hemispherical electron energy analyzer, electrons ejected from a sample are passed through an input lens, so that the electrons are decelerated and focused. Then, the electrons are made to enter the analyzer. In this case, two different modes of operation are used. In one mode, the ratio of the electron pass energy E.sub.p in the analyzer to the energy E of emitted electrons is always kept constant. This sweeping mode is hereinafter referred to as the CRR (constant retarding ratio) mode. In the other mode, the pass energy E.sub.p is always kept constant, irrespective of the emitted energy E. This mode of operation is hereinafter referred to as the CAE (constant analyzing energy) mode. This is, in the CRR mode, the E.sub.p varies in a constant relation to energy E, for example, when the E is 100 eV, the E.sub.p is 10 eV, and when the E is 200 eV, E.sub.p is 20 eV. On the contrary, in the CAE mode, the E.sub.p is always 10 eV, whatever the E is changed.
In Auger electron spectrometry, the CRR mode is often used. Where plural detectors are regularly spaced from each other, if the instrument is operated in the CRR mode as the swept energy is varied, the difference between the detection energies of the successive detectors also varies proportionately. Therefore, even if the energy is stepped in equal increments for the detector acting as a reference detector, the energy is stepped in unequal increments because of the variation of the detection energy difference between the reference and the other detectors. For example, if the energy is stepped in increments of 1 eV, e.g., in the manner as 100 eV, 101 eV, 102 eV, and so on, for the reference detector, it does not step likewise for the nonreference detectors. The detection energies will be, for example, 101.01 eV, 102.03 eV, and so on. In this way, the energy is stepped in unequal increments and by this the detected energies involve fractions. Since the energies detected by the detectors cannot be made coincident with each other as mentioned above, the values counted by the detectors cannot be summed up at the same detection energy. This induces the degradation of accuracy for the finally processed data.
In photoelectron spectrometry, the CAE mode is frequently used. In this case, the pass energy E.sub.p is kept constant as described above. Therefore, the difference of detection energies between one detector and the reference detector remains constant. Thus, if the reference detector is stepped in equal energy increments, then the other detectors are stepped in equal increments. In consequence, the problems appearing in the CRR mode do not take place. In this case, however, if the energy increment E.sub.inc is not an integral multiple of the difference between the energies detected by the successive detectors, then the consistency will be lost when values counted by the detectors are summed up. In this way, a restriction is imposed on the setting of the increment E.sub.inc.
As seen above, if the multidetector scheme is directly applied to the prior art electron energy analyzer, the two requirements for the normal multidetection cannot be met in the CRR mode, and the same restriction for the selecting energy step will be imposed in the CAE mode. Accordingly, the multidetector schemes have been applied in a very limited condition.